Chlorine is produced commercially in electrolytic cells in which an aqueous solution of sodium chloride is reacted to yield chlorine gas at the anode.
Various cell designs are employed and one in general use in the United States is the diaphragm cell. In this cell, the anode is titanium with a surface coating of an electrocatalytic material such as ruthenium dioxide or a platinum-iridium alloy. The cathode is mild steel in the form of a perforated sheet or a wire screen. Anolyte or catholyte zones of the cell are separated by a diaphragm and this is usually made of asbestos which is applied as a slurry to the cathode before the cell is assembled.
In a diaphragm cell, saturated brine is fed continuously to the anolyte compartment and the resistance to flow due to the membrane creates a hydrostatic head in the anolyte. A direct current is passed through the cell and chlorine is formed at the anode while sodium hydroxide and hydrogen are formed at the cathode. Product chlorine fills the head space of the enclosed cell and is removed by piping to a header. Hydrogen can not be permitted in the head space and is removed through the perforations or other openings in the cathode along with the spent brine.
Sodium chlorate is produced in a somewhat similar manner. In a sodium chlorate cell, the anode is titanium with the same electrocatalytic coating used for chlorine generation. The anode is usually a flat sheet of titanium and the cathode is usually a flat sheet of mild steel or titanium. There is no diaphragm. Saturated brine is fed continuously to the cell and under the influence of a direct current, sodium chloride is formed in solution at the anode. Hydrogen is formed at the cathode according to the overall reaction EQU NaCl+3H.sub.2 O+6 Faradays NaClO.sub.3 +3H.sub.2.
In design of sodium chlorate cells, it is common to use the electrodes in a vertical position and to place them in close parallel arrangement, typically 0.5 inches or less apart. Hydrogen gas is envolved in large amounts and as it rises to the head space of the cell, this gas serves to agitate the electrolyte and contributes to the efficiency of the reaction. Product sodium chlorate is removed in solution in the spent brine.
In chlorine or chlorate cells of the type described, mild steel cathodes have advantages of low cost and low overpotential for hydrogen formation. They have the disadvantage that in the absence of a potential, chemical corrosion by hot, saturated brine is rapid. In the large plants operated by the major producers (DOW, PPG, Hooker, Pennwalt) it is common to have a stand-by cathodic protection system with its own generator to supply a small potential whenever the main power is off for any reason.
In contrast to mild steel, titanium cathodes are not chemically attacked by hot, saturated brine. While the cost of titanium is substantially higher than that of mild steel, elimination of the need for a stand-by cathodic protection system makes titanium an attractive cathode material and titanium cathodes are used, principally in some sodium chlorate cell designs. Titanium has a disadvantage in that its hydrogen overpotential is higher than that of mild steel, by approximately 0.2 volts. Cells operate at voltages of 3.2 to 4.0 volts with 3.5 volts being typical. Amperage is held constant since this determines plant production. An increase of 0.2 in cell voltage means an increase in power consumption of about 6%. Since power consumption represents half or more of the selling price of chlorine or sodium chlorate, a 6% increase is substantial.
A second disadvantage of titanium cathodes is that titanium metal reacts with hydrogen to form titanium hydride. Over a period of time (12 to 18 months) the immersed portion of a titanium cathode becomes saturated with titanium hydride. The portion of the cathode above electrolyte level has little or no titanium hydride. This concentration variation causes internal stresses which are sufficient to warp the cathode to a degree that it can touch the anode causing arcing and destruction of both electrodes. Cells which use titanium cathodes are usually shut down periodically for cathode inspection and replacement when hydride formation has reached an advanced state.
Since reduction of hydrogen overpotential (overvoltage) is economically important, earlier workers have developed coatings for cathodes to achieve this. For example, U.S. Pat. No. 3,974,058 describes steel cathodes having an intermediate coating of cobalt and an overcoating of ruthenium. U.S. Pat. No. 4,000,048 describes titanium cathodes which are coated with palladium-silver or palladium-lead alloys to lower hydrogen overpotential and to reduce hydrogen embrittlement of the titanium substrate (column 2, lines 39-46).
A novel approch which does not necessarily involve a coating is described in U.S. Pat. No. 4,075,070. These workers have found that a titanium alloy containing a very small amount of rare earth (preferably Ti-0.02% Y) has a hydrogen overpotential lower than that of pure titanium or certain other alloys of titanium. In addition, these workers have found that the preferred alloy has slower uptake of hydrogen as measured by weight gain in laboratory tests.